In a gas turbine engine or other engine with gearbox containing journals, bearings or other lubricated moving components, high speed motion requires that lubrication be provided consistently. For example, the interruption of oil supply to journals of a high speed gas turbine engine for only two seconds can cause seizing of the bearings and catastrophic failure of the engine. Oil flow for cooling and lubrication must be continuously supplied to journals, gears and bearings, however, oil flow can be interrupted by negative gravity forces, air bubbles in the oil supply lines or temporary malfunction of valves, pumps and filter assemblies. In order to overcome any temporary interruption in oil flow, the prior art includes an oil accumulator within the high pressure oil line downstream of the oil pump to accumulate a volume of oil under pressure, that can be deployed during a temporary interruption.
The provision of an oil tank together with an external accumulator however involves several disadvantages. In aircraft applications especially, the weight and space taken by an oil accumulator is often considered unjustifiable. The added risk of leakage, extra piping and weight of an external accumulator with check valve to separate the high pressure accumulator oil volume from the low pressure oil tank volume is considered by many designers to be unjustified.
In addition, when an external accumulator is provided, the oil tank must be oversized to compensate for the extra oil used in charging the accumulator. When an engine is initially started up, the oil pump withdraws oil from the oil tank, pressurises the oil, and delivers oil to an accumulator. The accumulator fills with pressurised oil and thereafter distributes lubricated oil to the necessary rotating engine components. On engine shutdown, the oil pump ceases operation and the pressurized oil from within the accumulator is released to the gearbox and journals then scavenged and returned to the oil tank. Therefore, when an accumulator is used there is a disadvantage in that the oil tank must be sized to accommodate the extra oil within the system. A further disadvantage of an external accumulator is the variation in oil level between the operating condition with oil stored in the accumulator, and initial start up condition with oil tank level full and accumulator oil level reduced. Therefore, use of an accumulator not only adds the weight and volume of the accumulator and associated piping, but also increases the necessary size, weight and volume of the oil tank to accommodate an increased volume of oil circulating within the oil supply system.
It is an object of the invention to provide a combined oil tank and accumulator in such a manner that the volume of oil circulating within the oil supply system is not increased and wherein the oil level within the oil tank remains relatively constant while providing an accumulator to compensate for oil supply interruption on a temporary basis.
It is a further object of the invention to provide a combined oil tank and accumulator wherein oil level is maintained relatively constant between start and running conditions.
Further objects of the invention will be apparent from review of the disclosure, drawing and description of the invention below.